1. Field of the Invention
This invention relates to a Plasma Display Panel, hereinafter referred to as a PDP, which is a kind of thin display devices.
2. Description of the Related Arts
PDPs excel in the visual sensation because PDPs are of a self-luminescent type, and is comparatively easy to accomplish a large and high-speed display which suits television displays. Especially, surface discharge type PDPs are suitable for a color display by the use of fluorescent materials.
Large screen size is one of demands from the market for the PDP. In order to satisfy this demand, the development of the structure and the PDP manufacturing method suitable for a large panel has been progressing.
PDPs have discharge spaces therein arranged on a substantially flat plane. The panel envelope to form the external outline is provided by a pair of substrates opposed from each other via the discharge space. At least the substrate on the front side must be transparent. Soda lime glass plates are usually used for the substrates on the front side and the back side.
In a PDP display method where a lot of discharge cells arranged in a matrix emit lights selectively, there are separator walls, which are often called ribs, to define the discharge spaces.
The height of the separator walls is equal to the gap clearance of the discharge spaces. For instance, in a surface discharge type PDP where the display electrodes to form the discharge electrode pairs are arranged in mutually adjacent and parallel relationship, the separator walls lie straight on a plane and are provided at equal intervals in the direction of the line of the display, i.e. in the direction along which the display electrodes extend. Spread of the discharge is limited by the separator walls, whereby discrete discharge cells are defined. Accordingly, an accurate matrix display is accomplished.
Moreover, the separator walls play the role of distance pieces, i.e., spacers, to provide equal gap clearance of the discharge spaces all over the display area, in which an unequal clearance may affect the discharge condition.
The manufacturing process of a PDP is divided roughly into three processes. That is, PDP is completed after sequentially undergoing a process by which predetermined composition elements are formed on each substrate so as to make the front panel and the back panel, a process in which the front panel, and the back panel thus made respectively in this manner, are combined (sealed) with each other, and a process to fill a discharge gas therein after cleaning the inside. Usually, the front panel and the back panel are manufactured in parallel.
Main composition elements in surface discharge type PDPs are, for example, display electrodes, a dielectric layer for the AC drive, a dielectric layer protection film, electrodes for addressing the discharge cell to be lit, separator walls, and fluorescent material layers.
The formation of these composition elements accompanies heat processes. For example, in forming the display electrodes, the substrate is heated at a sputtering or vacuum evaporation of a film forming process of the conductive layer. Moreover, in forming the dielectric layer, a thick film material, represented by a low melting point glass, is heated so as to melt.
In forming plural composition elements sequentially on the same substrate, in the prior art, the material and the heat process condition of each composition element were selected so as to allow no influences, such as the deformation or change in quality, on the previously formed composition elements. For example, in the case where the heating is performed two times, the heating temperature of the second time is chosen lower than the heating temperature of the first time; and accordingly, the materials to be heated are chosen to correspond to the required heating temperatures.
In manufacturing PDPs as mentioned above, whenever the composition element is formed the substrate is expanded and contracted. Therefore, in mass-production, most substrates are in a warped state when each panel is finished, even if a smooth substrate is employed for the front panel or the back panel. The warp of the substrate becomes remarkable as the PDP screen size, i.e. the outline dimension of the substrate, becomes larger.
In prior arts, the direction of the warp of the substrate was irregular. That is, sometimes the inner surface on which the composition elements have been formed becomes convex, which is referred to hereinafter as a xe2x80x9cwarp in a positive directionxe2x80x9d; or sometimes, on the contrary, the warp is such that the inner surface becomes concave, which is referred to hereinafter as a xe2x80x9cwarp in a negative directionxe2x80x9d. Therefore, there were problems as follows.
FIGS. 1A to 1C schematically illustrate a cross-sectional view of panel shapes in the prior art sealing steps. In FIGS. 1A to 1C, there are partially omitted the composition elements in order to make the figure simple, and the warp of the substrate is exaggerated.
The problem of the prior arts are hereinafter explained together with the procedure of the sealing process. A glass substrate 110 having a display electrode 120 thereon and a glass substrate 210 having plural separator walls 290 thereon are sealed with each other. Prior to the sealing operation, low melting-point glass layers 310 as the sealant are placed on the edges of glass substrate 210, the thickness of the low melting point glass layers 310 being chosen to be higher than the height of separator walls 290.
Glass substrate 110 and glass substrate 210 are stacked with each other as shown in FIG. 1(a). The pair of glass substrates 110 and 210 is heated while pressed to each other so that low melting-point glass layer 310 is melted. Subsequently, the substrate temperature is lowered so that glass substrate 110 and glass substrate 210 are sealed with each other as shown in FIG. 1B.
If there is a warp in a negative direction on glass substrate 110 at the time of starting such sealing process, a gap g is undesirably created between separator walls 290 and the inner surface of the glass substrate 110 unless a warp in a positive direction to counter the warp of the glass substrate 110 is on the opposite glass substrate 210 having separator walls 290. In the example of FIGS. 1A and 1B the gap g is created because glass substrate 210 is flat.
When a PDP is completed after a discharge gas is filled therein as shown in FIG. 1(c), the warped state is such that the central portion of glass substrate 110 is depressed due to a low internal pressure of about 500 Torr. (=66,700 Pa), which is lower than the standard atmospheric pressure 760 Torr (=101,325 Pa). The gap g does not completely disappear even though the deformation of glass substrate 110 allows the gap to become smaller than that at the beginning of the sealing operation. Therefore, there was a problem in that the display fell into disorder by the generation of so-called cross-talk caused from an excessive spread of the electrical discharge through the gap g between the substrate and the top of separator walls.
Moreover, when the degree of the warp of the glass substrate was large, there was another problem in that the glass substrate cracked at the sealing process, or cracked afterwards during the step of connecting an external driving circuit thereto, that is connection of flexible cable by an application of mechanical pressure thereover.
In addition, even in the case having no gap g, if the PDP is used in an environment where the external air pressure is lower than the standard atmospheric pressure, the center surfaces of glass substrates 110 and 210, defining the panel envelope, projected toward the outside to cause the increase in the substrates"" gap, resulting in the gap g between the substrate and the top of separator wall. That is, the problem was also in that the atmospheric pressure range in which the PDP can properly operate was limited.
It is a general object of the present invention to provide a plasma display panel to accomplish a high quality display, wherein no gap is existing between the top surface of the separator walls and the inner surface of a glass substrate opposing the other, so that the discharge spaces are correctly defined.
It is another object of the present invention to decrease the damages of the substrates so as to raise the yield of the production.
It is a further object of the present invention to expand the range of atmospheric pressure in which the PDP operates correctly.
In a PDP according to the present invention, the front substrate and the back substrate are respectively in a warped state such that a central portion of each substrate projects in a frontwards direction relatively to a peripheral portion of the substrate, so that the front surface is convex.
After the panels are sealed with each other, a stress remains in the substrates such that the two substrates are pressed to each other with an elastic deformation.
In the finished PDP, a height difference of the central portion measured from a central part of a short side of a substrate divided by a longitudinal width of the substrate is preferably less than 0.1% for the front substrate and the back substrate, respectively.
In preparing the two substrates, the front panel and the back panel are respectively warped towards each other so that the facing inner surfaces are convex during the process of being sealed with each other. A height difference ratio of the central portion from a central part of a short side of the back substrate is preferably less than 0.16%. A height difference ratio of the central portion from a central part of a short side of the front substrate is preferably less than 0.06%. A difference of the height difference ratios of the back substrate and the front substrate is preferably in the range from 0 to 0.1 percentage point.
Owing to this remaining stress, the gap between the separator walls and the inner surface of the facing panel is correctly maintained even in an external air pressure lower than the internal pressure of the PDP.
The above-mentioned features and advantages of the present invention, together with other objects and advantages, which will become apparent, will be more fully described hereinafter, with references being made to the accompanying drawings which form a part hereof, wherein like numerals refer to like parts throughout.